Abstract : An investigation has been undertaken to study the nature of cumulative damage occurring in glass epoxy composite test coupons and sub-structures during fatigue loading, and to correlate the extent of this damage with remaining useful lifetime. Changes of stiffness in composite laminate specimens loaded in tension-tension fatigue at various load levels have been shown to reflect the extent of accumulated damage and the magnitude of the stiffness change has been used to predict the remaining life fraction of the specimens. Dynamic structural analysis techniques can be used to follow these stiffness changes in substructures, and should be useful as a field technique for following stiffness losses in composite structures. The nature of the damage which occurs during fatigue loading has been characterized by a number of nondestructive evaluation (NDE) techniques including ultrasonics and infrared thermography.